Add debug target to makefile.

[Stewori]

In #10 it was explained how to build JyNI in debug mode. That requires editing the makefile so we should better have a build target for debug. I created this issue to keep track of that idea.

[CalumFreeman]

I've added a basic debug build target, it just compiles with the -g flag which adds debug symbols to the file (I also added a tests target that just prints a message saying it isn't implemented yet). The pull request is 31.

[Stewori]

Alright, that's a good start. Even more elegant than what I had in mind -- I wasn't aware of += notation for makefiles. According to https://clang.llvm.org/docs/UsersManual.html#controlling-debug-information the -g flag should also work for clang so we can readily apply the same enhancement to makefile.clang and makefile.osx. For msvc we probably want /Zi, see https://msdn.microsoft.com/en-us/library/958x11bc.aspx. However it might be necessary to handle the .pdb files that are created in addition to the .obj files then. E.g. for clean up, but maybe they also need to be passed to the linker, I'm not sure right now.
@CalumFreeman It's fine if you focus on Linux now, the notes on other OS' are just because in the long run we should be able to have debug builds there as well. I'll leave the issue open until we found solutions for the other makefiles.